Hot Jupiters: The Metallicity Question

by Paul Gilster on January 29, 2007

A globular cluster is a glorious thing. Consider Omega Centauri, a vast city of stars about 15,000 light years from Earth. Clusters like this one are composed of millions of Population II stars, meaning they’re among the oldest observed stars and may date back as far as twelve billion years. A result of their early formation is that they remain deficient in metals (in astronomical terms, the elements above hydrogen and helium), making them ideal laboratories for a particular branch of exoplanet studies.

Image: This image of Omega Centauri, the brightest and largest globular cluster in the sky, was obtained with the Danish 1.5 m telescope at the ESO La Silla observatory. It shows the central part only; the cluster is actually much larger than the field reproduced here. Credit: European Southern Observatory.

A growing assumption about the massive ‘hot Jupiters’ we’ve found in our early planet hunting is that their existence depends upon a relatively high metallicity in their host star. The planets we’ve found certainly fit that bill, but is this just an observational bias, given that we’re looking at nearby stars that are all metal-rich to begin with? To test the idea, a team led by David T.F. Weldrake (Max-Planck Institut für Astronomie, Heidelberg) has been looking at Omega Centauri and 47 Tucanae, the two brightest clusters in the sky, to see if metal-poor stars produce such planets.

Working with Australian National University’s 1-meter telescope and Wide Field Imager, the team compiled 56 nights of data and gathered photometry on 53,000 main sequence stars, sampling the outer halos of each cluster. Analyzing the occurrence frequencies expected for gas giants in close orbits and factoring in the transit probabilities, the team expected to find 7 planets in 47 Tucanae and 5.3 in Omega Centauri. The actual result: not a single transit around any of the sampled cluster stars.

Note the method here. The work proceeded under the assumption that stellar metallicity does not affect the occurrence of these planets in globular clusters. If that were the case, the numbers expected should have been found. The result is statistically significant. From the paper:

…our null result suggests that stellar metallicity, not dynamics, is the dominant effect limiting the frequency of short period massive planets in globular clusters, and places an observational constraint on planetary frequency at such a low metallicity. Perhaps the low metallicity does not affect planet formation, but does affect planetary migration. If true, then long period planets should still exist in these clusters, undetectable in our work.

Centauri Dreams‘ take: The upshot is that we have to be careful in stretching our conclusions too far. What we can say is that the existence of metals in stellar atmospheres is implicated as the leading factor in preventing hot Jupiters from forming in these clusters. But the other great imponderable, how such planets migrate to the inner system, is still in play. Its possible role in planetary formation needs further study.

And because we’ve speculated in these pages about the view from a terrestrial world in a globular cluster, let’s add one more thing. While the planet/metallicity connection seems to be firming up rather well for gas giants, we know little about its effect on smaller, rocky worlds. It seems a natural assumption that these glorious cities of stars would be bereft of such planets, but the present work does not address that question.

The paper is Weldrake et al., “Searching for Planetary Transits in Globular Clusters – 47 Tucanae and ω Centauri,” slated to appear in the PASP proceedings of “Transiting Extrasolar Planets Workshop” (Heidelberg), and available as a preprint online.

I agree that we ought to take this data with a healthy grain of salt. The low-numbers of expected events barely scratches the surface of “statistical significance”.

Additionally, metallicity is not the only factor at play in globular clusters. Stars within globular clusters are more likely to be multiples and much, much more likely to have many close encounters with other stars over the course of their lifetimes. Both of these factors would tend to depopulate planetary systems. Ultimately I think the chaotic history of globular cluster stars all but rule out their use in answering fundamental questions about planetary formation, we must look to stars where we can isolate the variables related to planetary formation and retention much better than we can with this population of stars.

The point of this study was that it was done in the outer regions of the star clusters where the dynamic effects are thought to be less important.

There was a similar study done on the central regions of 47 Tuc back in 2000 I think. Of course they didn’t find any planets at all either but they couldn’t really decide if it was due to the low metallicity of the stars or due to dynamical effects.

Search for “a lack of planets in 47 Tucanae from a hubble space telescope search”. The abstract said:

“[…]The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.”

So this study more or less decides that the cause was low metallicity.

BTW the study gives 3.3 sigma and 2.8 sigma signifance for the two clusters, so I’d say the two results combined are pretty significant.

Id still like to see the results from metal-poor stars that arent in any part of a cluster.. you never know what kind of variables might be at work. if they got the same results then, that would definitely be statistical signifigance.

Hi Everyone,
First of all, thank you to the creators of this website for discussing our recent result on omega cen. Im very pleased that it is receiving a good press.

In response to the last entry above, we are attempting to answer this question by studing a random galactic field with the same instrument and methods, kind of a control field for the globulars to compare the results.

We’ve found some interesting results with the variable stars that may be telling us something about the cluster’s evolutionary histories, but more importantly we have found several transiting planet candidates, including one excellent candidate, in direct contrast to nothing in either cluster. We’ll figure out what the candidates are later in the year, but this is another piece to the puzzle, and also indicates that globular clusters have fewer short period giant planets than the immediate solar neighbourhood.

Very interesting work. A couple of points. Its pretty much accepted that
in globular clusters, core collapse is averted through the creation of tight binaries that form from the interaction three or more bodies. In other words, while some bodies will be ejected from the core, dynamical considerations could still favour the creation of hot jupiters. If for example the “free floating planets” that have been seen in a couple of star forming regions also existed when the globular clusters were formed. So maybe the non-detection of hot jupiters in these globular clusters is also telling us something about the initial mass function.

Abstract: We argue that all transient searches for planets in globular clusters are hopeless. Planets in low metallicity stars typically don’t reside at small orbital separations. The dependance of planetary system properties on metallicity is clearly seen when the quantity of the planet mass times periastron distance square (Ie) is considered. In high metallicity systems there is a concentration of systems at high and low values of Ie, with a low populated gap near Ie=0.3 (in units of Jupiter mass times AU square). In low metallicity systems the concentration is only at the higher range of Ie.

Therefore, it is still possible that planets exist around main sequence stars in globular clusters, although very rarely because of the low metallicity, but at orbital periods of >10 day. We discuss the implications of our conclusions on the role that companions can play in the evolution of their parent stars in globular clusters, e.g., influencing the distribution of horizontal branch stars on the Hertzsprung-Russell diagram of some globular clusters.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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